In a gas turbine engine, the burning of fuel takes place within a combustor. As is shown in U.S. Pat. Nos. 4,077,205 and 4,380,906, some combustors are comprised of a plurality of annular, mating louver details or members. Each louver member has a plurality of circumferentially disposed holes therein to admit a film of cooling air into the interior of the combustor. This film of cooling air limits the temperature to which the louver members are heated by hot gases formed during the combustion process.
It is essential for optimum cooling effectiveness that this film of air, which forms a protective boundary between each louver member and the hot stream of combustion gases, be as continuous as possible. As a result, designs have been proposed in which the cooling holes are closely spaced apart, and each hole as a very small diameter.
One technique for forming such holes uses a high energy beam, preferably a laser beam. In a hole piercing operation using a laser beam, the individual louver member is rapidly moved, or indexed, into position under the beam, then a shutter placed between the beam source and louver member opens for a predetermined interval to allow the beam to impinge upon the member and pierce the hole. By this method, many holes may be pierced in the louver member in a short period of time. However, it has been found that such rapid rate piercing of small diameter holes sometimes results in the formation of cracks which initiate at the hole surface and propagate into the substrate material. It is believed that the cracks may result from thermal shocking, which occurs as a result of the rapid laser piercing operation. Due to the severe environment in which a combustor in a gas turbine engine operates, these cracks should be minimized, in order to avoid an unacceptable debit in mechanical properties. The present invention results from a development program to define procedures for the laser piercing of closely spaced, small diameter holes in wrought, nickel base superalloy louver members without the formation of such cracks.